I'm planning to install an engineered hardwood floor on a concrete slab that is very very cold. The climate is northern New Mexico. The slab perimeter is not insulated, and doing so is cost-prohibitive at this time as it requires removing more concrete that abuts it, some of which is structural. The slab's thermal mass is not contributing much if anything to moderating internal temperature conditions due to the house's siting being unable to take advantage of much direct solar irradiance. All it's doing is making the house cold.
So I'm thinking of applying polyiso foam insulation board over the top of the slab but below the new floor, basically cutting off the thermal mass of the slab from the interior climate. If I were to do this, where should the moisture barrier go? Between the bare slab and the insulation board? Between the insulation board and the wood floor, perhaps with a moisture-barrier-type underlayment? Would I even need a moisture barrier at all with this setup?
Best Answer
I am curious what you consider to be "very very cold". Here in Maine, 45 to 50 degrees at slab level 4 feet below grade is common. So interested in what temps you may have and the average relative humidity in that space. With those questions asked; the straight answer to your question is yes, you should use a vapor barrier when installing any wood type floor over concrete. Both methods you mentioned would work. A one inch foam insulation, closed cell, waterproof, under the subfloor would be fine. If you use the tongue and grove style and fit it carefully, additional vapor barrier should not be needed. You can add a sheet of 6 mil plastic under the insulation for good measure if you feel the humidity is high in your area and are concerned with condensation. Another option is a paint on barrier such as Hydra-loc or any low pressure concrete coating used for interior waterproofing. Again, use this under your foamboard insulation. The one step method is the interlocking subfloor systems with a built in moisture barrier. this works well but is very pricey.